Advances in microelectronics often are limited by the multitude of relatively complicated processing steps required to produce the devices. A typical example of the number of processing steps is apparent in the trench isolation technique in bulk silicon which has been investigated as a means of dielectric isolation. This technique requires etching of deep trenches between devices on the die followed by oxide growth in the trenches to form the dielectric isolation. The etching steps call for the application of a photoresist with a subsequent low temperature heat treatment. This is followed by exposure to a lamp through a mask in contact with the photoresist and development of the resist. Another heat treating step is next, then the silicon trenches are chemically etched and, lastly, the remaining photoresist subsequently is stripped from the silicon. These seven steps are typical in many standard etching techniques used in the semiconductor industry. It becomes apparent that significant savings and yield could be obtained through the more simplified procedures that might be provided by a maskless, contactless form of etching. The reduced complexity of such a procedure would eliminate the many time consuming and costly steps of the conventional etching technique.
Another particular example of the excessive number of processing steps required to produce microelectronic devices becomes apparent when noting the procedures used to fabricate a backside illuminated charge-coupled device (CCD). CCDs designed for solid-state cameras, such as camcorders, are in great demand and are widely available. They have been designed to provide adequate performance when viewing brightly illuminated scenes. However, in astronomical, scientific and military applications their spectral response, readout noise, dark current, full well-capacity and blooming characteristics are not satisfactory.
To overcome the limitations of imaging through the polysilicon gates that necessarily cover all of the sensitive pixel array, it would be desirable to illuminate the CCD from the backside if the silicon substrate were thin enough. In other words, a solution to obtaining better light sensitivity would be the thinning of the backside of the CCD to a total thickness of roughly 10 microns. The need is quite apparent for new microelectronic processing schemes to produce thin membranes such as those required for the backside illuminated CCDs. Additional features that should attend this thinning process are the creation of a smooth surface for uniform imaging, nonreflecting sidewalls for stray light rejection and large (approximately 2 mm by 2 mm) square cross section for optimal illumination of the active area of the array.
A conventional fabrication procedure for backside illuminated CCDs calls for chemical thinning of the silicon. However, the standard wet chemical thinning-etch procedure produces an extremely low yield process and requires the handling of fragile thin silicon membranes. Furthermore, the chemical thinning requires two processes, a deep etch using potassium hydroxide and a subsequent Dash polishing etch. The latter consists of applying a mixture of acetic, nitric and hydrofluoric acids along with a surfactant. The Dash etch process also requires additional masking to protect the frontside metalization and backside gold eutectic used for packaging. Additional cleaning and inspection steps are required to complete the thinning process. Elimination of these steps would allow further "dry" processing of the thinned die, such as laser doping or dopant activation. In addition, the minimization of the required number of processing steps always is desirable in this microelectronic processing procedure to maximize the yield and reliability while also reducing costs.
In view of the foregoing, noncontact, maskless processing is receiving widespread interest in the microelectronic industry. A variety of laser-assisted processing techniques to modify materials used in this industry are being pursued, particularly with the introduction of the excimer laser which typically emits at the shorter wavelengths. The works of D. Ehrlich et al. in their article "A review of Laser-Microchemical Processing" J. Vac. Sci. Technol. B., 1, 969 (1983), F. Houle, in her article entitled "Basic Mechanisms in Laser Etching and Deposition" Appl. Phys. A, 41, 315 (1986), D. Bauerle in the article entitled "Chemical Processing with Lasers: Recent Developments" Appl. Phys. B, 46, 261 (1988), and T. Chuang in the article entitled "Laser-Induced Chemical Etching of Solids: Promises and Challenges" in A. Johnson et al., ed's, Laser Controlled Chemical Processing of Surfaces, Materials Research Society Symposia Proceedings, Vol. 29 (New York: North Holland, 1984), pp. 185-194, offer a review of the effort involved with laser-assisted processing techniques. As a consequence, laser processing has grown from a purely research effort into a production tool. Early on, however, investigations related to laser processing of silicon led to the conclusion that laser ablation of silicon using an excimer laser was considered undesirable since the surface quality would be poor, although the rate of material removal would be high.
The use of halogens to etch silicon is well established by the text of the Gutmann, Halogen Chemistry, Vol 2 (New York: Academic Press, 1967), pp. 173-180. In addition, an existing body of research for plasma processing of silicon offered another group of candidate etchants as described by H. F. Winters et al. in the article "Surface Processes in Plasma-Assisted Etching Environments" J. Vac. Sci. Technol. B, 1, 469 (1983) and B. A. Heath et al. in the article "Plasma Processing for VLSI" chapter 27 in M. G. Einspruch, ed. VLSI Handbook (San Diego: Academic Press, 1985) pp. 487-502.
The laser-assisted etching of silicon has been examined using a chlorine ambient by R. Kullmer et al. in their article "Laser-Induced Chemical Etching of Silicon in Chlorine Atmosphere: I. Pulsed Irradiation" Appl. Phys. A, 43, 227(1987), P. Mogyorosi et al. in the article entitled "Laser-Induced Chemical Etching of Silicon in Chlorine Atmosphere: II. Continuous Irradiation" Appl. Phys. A, 45, 293 (1988), R. Kullmer et al. in the article "Laser-Induced Chemical Etching of Silicon in Chlorine Atmosphere: Combined CW and Pulsed Irradiation" Appl Phys. A, 47, 377 (1988), Y. Horiike et al in the article "Excimer Laser Etching on Silicon" Appl. Phys. A, 44, 313 (1987) and W. Sesselmann et al. in their article entitled "Chlorine Surface Interaction and Laser-Induced Surface Etching Reactions" J. Vac. Sci. Technol. B, 3, 1507 (1985). S. Palmer et al. in their article entitled "Laser-Induced Etching of Silicon at 248 nm in F.sub.2 /Ne" Conference on Lasers and Electro-optics Technical Digest Series 1988, Vol. 7, 284 (Optical Society of America, Washington, D.C., 1988) examined the fluorine ambient.
The use of nitrogen trifluoride ambient was discussed by J. H. Brannon in his article entitled "Chemical Etching of Silicon by CO.sub.2 Laser-Induced Dissociation of NF.sub.3 " Appl. Phys. A, 46, 39 (1988) and the above referenced article by Y. Horiike et al. The use of the halogenated ambient xenon difluoride was discussed by T. Chuang, "Infrared Laser Radiation Effects on XeF.sub.2 Interaction with Silicon" J. Chem. Phys., 74, 1461 (1981), by F. Houle "Photoeffects on the Fluorination of Silicon: I. Influence on Doping on Steady State Phenomena" J. Chem. Phys., 79, 4237 (1983) by F. Houle in the article "Photoeffects on the Fluorination of Silicon: II. Kinetics of the Initial Response to Light" J. Chem Phys., 80, 4851 (1984) And, the use of halogenated ambient sulphur hexafluoride was examined by T. Chuang in his article entitled "Multiple Photon Excited SF.sub.6 Interaction with Silicon Surfaces" J. Chem. Phys., 74, 1453 (1981).
Typical etch rates of approximately one angstrom per pulse have been reported for the ambients of the preceding paragraphs under a variety of conditions. Such etch rates with the high pulse repetition rate of the excimer laser (100 Hz typical, 250 Hz available) were satisfactory to meet yield requirements of some applications. However, difficulties in handling and processing pure halogens such as chlorine and fluorine make them less suitable for inserting into existing manufacturing processes. Furthermore, pure halogens and many halogenated ambients are corrosive in nature and will spontaneously react with some (or all) of the materials composing a partially fabricated microelelectronic device. Masking, therefore, is required in such ambients despite the non-contact nature inherent in laser processing. Masking may be difficult or impossible in many applications. In addition, the detrimental effects of chlorine on the radiation hardness of silicon devices makes it potentially unsuitable for a wide variety of military or space applications. There also is evidence to at least suggest that the use of chlorine creates rough surfaces.
The use of laser-assisted wet etching was explored by R. Osgood Jr. et al. in "Localized Laser Etching of Compound Semiconductors in Aqueous Solutions" Appl. Phys. Lett., 40, 391 (1982), R. von Gutfeld et al in "Laser-Enhanced Etching in KOH" Appl Phys Lett , 40, 352 (1982) and F. Bunkin et al in "Laser Control Over Electrochemical Processes" SPIE Vol. 473, Symposium OPPIKA' 84, Vol. 473, pp. 31-37. The drawback to the laser-assisted wet etching technique is that it requires a different processing chamber to that of the gaseous "dry" etching technique and would require additional handling for further processing.
M. D. Armacost, S. V. Babu, S. V. Nguyen, J. F. Rembetski, in their article "193 nm Excimer Laser-Assisted Etching of Polysilicon", Mat. Res. Soc. Symp. Proc , Vol. 76, (1987), pp. 147-156, examine various ambients for etching polysilicon. They used two halocarbon ambients but found etch profiles that were not repeatable or did not show any appreciable etching. They did not examine the effects of etching silicon, nor investigate the critical parameters and processing windows required to achieve the results attained in accordance with this inventive concept.
The article by D. J. Ehrlich et al. "Laser Induced Microscopic Etching of GaAs and InP Appl. Phys. Lett., 36(8), Apr. 15, 1980 discusses laser induced etching of GaAs and InP in an ambient of methyl-halides (halocarbons) CH.sub.3 Br, CF.sub.3 I and CH.sub.3 Cl. The etching reactions discussed here depend on the absorption of the UV light by the gaseous ambient in order to dissociate the halogen atoms which react (etch) the sample. Therefore, in the case of InP etching by CH.sub.3 Cl, a minimal etch rate was observed (impractical for applications) because of weak absorption of CH.sub.3 Cl at the UV laser wavelength used. This invention does not require absorption of the UV light by the ambient to cause the chemical.reaction, in fact, absorption by the ambient is detrimental in the process of this invention because it minimizes the amount of energy impinging on the sample necessitating additional laser energy to initiate the reaction. This paper does not discuss etching in chlorofluorocarbon ambients, pyrolytic (thermal) dissociation of the ambient, nor exhibit etch rates as large as those disclosed in this invention. The article by Peter Brewer et al. entitled "Photon-Assisted Dry Etching of GaAs" Appl. Phys. Lett, 45(4), Aug. 15, 1984 discusses laser induced etching of GaAs in a CH.sub.3 Br or CF.sub.3 Br ambient. The reaction discussed requires absorption of the UV light by the ambient resulting on photolytic decomposition. As discussed herein, this differs from the pyrolytic decomposition method and apparatus of this invention and would, in fact, have a detrimental effect in the process of this invention. Furthermore, the layer etch rates represented in this article correspond to ablation of the sample causing significant materials damage and roughening at large laser energy densities (fluences). Variations in etch rate were found for different crystal orientations in contrast to no crystal orientation difference in the method of this invention. The article by Y. Rytz-Froidevaux et al. entitled "Laser Generated Microstructures" Appl. Physl. A., 37, 121-138 (1985) reviews laser-assisted deposition and etching processes on a variety of materials. Both III-V and II-VI semiconductor compounds are discussed using aqueous solutions as well as gaseous methy-halides and alkali halide salts. These reactions are photolytic in nature, as discussed above and, therefore, the absorption of the laser light would be detrimental in the process of this invention.
Thus, there is a continuing need in the state of the art for a maskless and contactless technique utilizing an excimer laser to promote a chemical reaction between a chlorofluorocarbon ambient and III-V semiconductor compounds gallium arsenide, GaAs, or indium phosphide, InP and their related ternary and quaternary compounds and II-VI semiconductor compounds cadmium sulfide, CdS, or zinc selenide, ZnSe, and their related ternary and quaternary compounds that eliminates many standard processing steps and has the advantage of processing in a prepackaged and pretested device and that can be extended to applications requiring micromachining or other depth profile-reducing techniques.